This invention relates to ball sealers for plugging perforations in a pipe and more particularly to ball sealers which will selectively bridge across perforations that are receiving a disproportionately large amount of well treatment fluid being injected into a wellbore.
During the drilling and in the operation of oil, water, or gas wells, it is often necessary to treat the borehole or earth formations penetrated by the borehole with a variety of treatment processes including fracturing, acidizing, or the like where fluids and materials are pumped into the wellbore from the surface and thence through casing perforation openings downhole into earth formations.
In such treating operations, it often occurs that a disproportionately large amount of the treating fluid or pumpable material passes through one or more of the several perforations in the casing.
The flow of a disproportionately large amount of treating material through one or a few perforations in the casing may be attributable to the higher permeability of the formation adjacent to those perforations. If the treating fluid may be easily pumped through one or a few perforations, it is often impossible to pump enough fluid into the well to build up sufficient hydrostatic pressure in the wellbore to force fluid or treating material through the perforations communicating with less permeable formations or generally impermeable sections of the earth formations.
One solution to the above-recited problem involves temporarily plugging at least some of the perforations communicating with the permeable sections of earth formations during the injection of treatment materials so that the hydrostatic pressure in the wellbore is permitted to develop to the extent that treatment fluids and materials are forced into the less permeable sections of the earth formation through other perforations which remain open. Ball sealers have been developed in the industry for accomplishing this selective plugging process to solve this fluid loss problem.
These ball sealing elements are usually made of rubber or of a hard-core material surrounded by a resilient outer covering. The balls are inserted into the well as fluid is pumped through the perforations. The balls are carried along by the flowing stream of fluid and seat against the casing perforations through which the preponderance of fluid passes, i.e., those perforations communicating with permeable sections of earth formation. Once seated against a perforation, the ball sealer element plugs the perforation and is held in place by the pressure against it of the fluid in the casing to thereby prevent passage of the fluid in the casing through the plugged perforations. Such ball sealers are shown in U.S. Pat. No. 2,754,910, issued Jul. 17, 1956, to Derrick; U.S. Pat. No. 3,011,548, issued Dec. 5, 1961, to Holt; U.S. Pat. No. 2,933,136, issued Apr. 19, 1960, to Ayers et al.; and U.S. Pat. No. 4,702,316, issued Oct. 27, 1987, to Chung et al. U.S. Pat. No. 4,702,316 shows a ball sealer composed of a polymer compound covered with an elastomer.
One disadvantage to the ball sealers in the patents listed above is that the plugging ball or element becomes lodged in the perforation so that when hydrostatic pressure in the wellbore is reduced, the ball sealer remains positioned in the perforation. Thus, the formation adjacent such permanently sealed perforations is no longer in communication with the wellbore, which would not only prevent treating materials from reaching those portions of the formation, but would also result in a decrease in production from that portion of the well served by those perforations.
Another problem encountered with ball sealers is that perforations are not always round, and a spherical ball may not be effective to bridge across the perforation opening which may have been formed as an irregular opening or later becomes split or cracked as a result of stress and chemical action in the wellbore. In any event, ball sealers of a conventional, spherically fixed configuration do not effectively seal such irregular openings. U.S. Pat. No. 3,376,934 issued Apr. 9, 1968, to Bertram proposes a solution to such a problem by providing a partially spheroidal body and a flexible skirt of fluid impervious material attached to and extending outwardly about the body to overspread the wall surface adjacent the perforation. This apparatus also is subject to becoming deformed to the extent that it may become permanently lodged in the perforation to thereby permanently close off such opening. One solution to this permanent sealing problem is suggested in U.S. Pat. No. 4,716,964, issued Jan. 5, 1988, to Ertsloesser et al., wherein the ball sealer is made of degradable material. This system requires that the chemical environment of the wellbore be maintained compatible with the materials of which the balls are made. Degradation of the material is also dependent on the wellbore fluid chemistry.
It is, therefore, an object of the present invention to provide a sealer device which can be pumped under fluid pressure into plugging contact with wellbore perforations that are receiving a disproportionate flow of fluids and which sealer devices will then release themselves from such plugging contact upon decrease in fluid pressure in the wellbore.